Raw oil containing wax mainly comprises diesel oil, atmospheric gas oil (AGO), vacuum gas oil (VGO), white oil, hydrocracking tail oil, lubricating oil distillate, and so on, wherein said wax is long chain n-alkanes or long chain hydrocarbons with a small amount of short side chains, comprising long chain alkanes, long chain aromatics and alkylcycloalkanes with a small amount of short side chains. Raw oil containing wax, especially heavy distillate resulted from paraffin-based crude oil, comprises a lot of wax with a high solidifying point and poor fluidity rate at low temperatures. As a result, the wax will make the oil thicken or even solidify with the decrease of the ambient temperature during the storage, transport and usage processes, which will block the oil pipeline and disturb the oil supply to the engine to cause the engine to fail. Now, there are many methods for solving various problems caused by the solidification of wax in raw oil containing wax, the mainly method being dewaxing, comprising solvent dewaxing, catalytic dewaxing and isomerization dewaxing.
The solvent dewaxing method means removing the wax by the solubility of wax in a solvent, the defects of which are the difficulty in solvent selection, the waste of a large amount of solvent, its harmfulness to people's health and the pollution to the environment, the high cost of equipment investment and operation, and the restriction of product quality by raw material.
The catalytic dewaxing method means using the catalyst with the shape-selection cracking function to make the wax component in the distillate to be selectively and catalytically cracked to generate smaller molecular hydrocarbons. For example, U.S. Pat. No. 4,247,388 and U.S. Pat. No. 4,659,311 disclose the usage of the catalytic dewaxing method to remove the wax in the lubricating oil. The defects of such method are that as a large amount of macromolecular compounds with high value are converted into smaller molecular materials with low value, the yield of base oil is low, the loss of viscosity index is great, and the value of byproducts is low.
The isomerization dewaxing method means only the macromolecular wax is allowed into the unique channel structure of the catalyst containing molecular sieves to carry out the isomerization to generate isoparaffins so as to obtain the effect of selective dewaxing. The isoparaffins have lower solidifying points and lower pour points when compared with wax of the same molecular weight, and they remain in the heavy distillate. The isomerization dewaxing method can decrease the solidifying point. At the same time, it can make the raw oil to have a high viscosity index and improve the yield greatly when compared with the above two dewaxing methods. The isomerization dewaxing method aims to convert the wax with a high melting point to isoparaffins with low melting points. However, the melting points of alkanes with high isomerization degree are high, so the isomerization degree of wax molecular should be controlled. As a result, the acid properties and pore structure of acidic components and hydrogenation components in the catalyst are strictly required. Generally, the acidic components should have a pore structure with moderate intensity, large acid amount and space limitation function, and the active metal components should have a fast hydrogenation/dehydrogenation activity to prevent further isomerization and even cracking of tert-carbocations.
Now, there are a lot of reports about the isomerization dewaxing catalyst. For example, U.S. Pat. No. 5,990,371, U.S. Pat. No. 5,833,837, U.S. Pat. No. 5,817,907, U.S. Pat. No. 5,149,421, U.S. Pat. No. 5,135,638, U.S. Pat. No. 5,110,445, U.S. Pat. No. 4,919,788, U.S. Pat. No. 4,419,420, U.S. Pat. No. 4,601,993, U.S. Pat. No. 4,599,162 and U.S. Pat. No. 4,518,485 relate to the isomerization dewaxing technology, wherein the used acidic components are mainly mordenites, SAPO-11, SAPO-31, SAPO-41, ZSM-23, SSZ-32, TON molecular sieves and so on, TON molecular sieves being mainly ZSM-22, Nu-10, KZ-2, ISI-1 and so on. Although the above materials have acidic centers of moderate strength, pore structures matching with wax moleculars, obvious space limitation on multi-branched chain isomers, and can make the paraffins to be isomerized to a certain extent, most acidic centers are covered in the preparation of a catalyst. As a result, the acidity of the obtained catalyst is weak, and the activity and selectivity thereof are low. In addition, the stability of a catalyst is closely related with its acidity. A catalyst with high acidity has strong resistance to sulfonitriding poisoning and has good stability, but promotes side reactions more easily, such as cracking reaction.